Solid-state lithium-ion thin-film batteries (TFBs) that are configured with a Lipon (Lithium phosphorus oxynitride—a potent lithium-ion thin-film electrolyte having the formula of ˜Li3.1PO3.3N0.5) electrolyte exhibit acceptable internal cell resistances over a wide temperature range while showing electrochemical stability in contact with very reducing electrodes, such as the metallic lithium anode, and very oxidizing electrodes, such as a charged Li0.5CoO2 cathode at 4.2V versus Li/Li+. In addition, Lipon has one of the lowest electronic conductivities of all known room-temperature lithium-ion electrolytes, thereby providing Lipon TFBs with a 10+ year shelf-life and an extremely low capacity loss per year (<1%) under ambient conditions.
Currently known lithium compounds, however, have internal cell impedances that can become undesirably large when Lipon TFBs are operated below 0° C., and in particular when employed in the −40° C. range, such as required in military or aerospace applications and by other government agencies. This characteristic is due to the bulk ionic conductivity of the Lipon electrolyte and its charge transfer resistances at both the cathode and the anode interface becoming undesirably large at low temperatures. The inverse of the internal cell impedance, the current rate capability of a cell, for instance, may be rated at 50 mA at 25° C. in the 4.2-2.0V voltage window for a standard single-cell 1 in2 TFB while increasing to more than 2000 mA for a few seconds at 100° C., but may be limited to 0.3 mA at −40° C.
From these performance data points, there exists a need to equip TFBs with an alternative, improved solid-state lithium-ion thin-film electrolyte. Particularly, there is a need for an electrolyte that possesses an equal or better electrochemical stability than Lipon versus highly reducing and oxidizing electrodes while providing a substantially enhanced low temperature performance.